The present invention relates to optical communication systems and more particularly to systems and methods for handling failures in optical communication systems.
In order to accommodate increasing demands from Internet and other telecommunications traffic, optical communication links are evolving to carry higher and higher data rates over greater and greater distances. Wavelength division multiplexed (WDM) links are carrying greater numbers of more densely spaced channels on the same fiber. For example, a single optical fiber may carry 160 channels, each having a data carrying capacity of 10 Gbps, making for a total available capacity of 1.6 terabits per second. Roughly speaking, this is the information equivalent of more than 15 million simultaneous phone calls.
With such a large volume of traffic depending on the operational status of a single WDM link, assuring link reliability is of paramount importance. Various approaches have been developed. Some operate at the client layer and rely on rerouting and redundancy features associated with client layer protocols such as SONET and MPLS. Because of the large number of client layer paths to be rerouted in the event of the failure of a high-capacity WDM communication link, continued development of optical layer protection remains very important.
One optical layer approach referred to as channel protection protects optical channels individually. A redundant path is provided for each optical channel by, e.g., providing a splitter at the transmit end and a switch at the receive end. This approach is very expensive since all the protection equipment must be provided on a per-channel basis.
An alternative approach, referred to as multiplex section protection, protects all of the WDM channels as a group. The optical fiber and intermediate amplifiers of the link are duplicated. Typically, a splitter follows the final multiplexing stage at the transmit end and splits the WDM signal between the primary and backup links. A switch to select between primary and backup precedes the first demultiplexing stage of the WDM receiver system.
U.S. patent application Ser. No. 10/139,411 discloses a variant of multiplex section protection where channels are protected on a per-subband basis. Protection unit equipment is inserted between multiplexing stages at the transmit end and between demultiplexing stages at the receive end. This is here referred to as optical multiplex subband protection.
To support the optical multiplex section protection scheme and the optical subband multiplex section scheme, it is desirable to have accurate and rapid detection of failures along the link. Detection of failure should preferably be sufficiently rapid to support correction of the detected fault within 50 ms. The number of falsely detected faults should be very low, even over the course of many years of operation. The fault detection scheme should be compatible with either optical multiplex protection or optical multiplex subband protection and should be readily adapted to any band or channel plan. The protection scheme should also be compatible with the architecture of OADM sites where only a limited selection of optical channels are extracted from and inserted into the light flow.
There are various existing approaches to failure detection but they all have shortcomings when applied to optical multiplex section protection and optical subband multiplex section protection. One can check for continuity of an optical service channel used to carry maintenance information but since this channel is not typically amplified, failure of intermediate amplifiers is not typically detected.
In an approach not admitted to be prior art, one can also designate an in-band amplified channel for continuity checking but this channel may fail individually for reasons other than failure of the amplifiers or fibers carrying the protected band or subband. Furthermore, designating a particular channel for fault detection limits flexibility in modifying the channel plan to suit disparate applications.
Other approaches rely on modulation superimposed on the individual channels to overcome Brillouin scattering effects. This modulation is referred to as a Brillouin tone. One could conceivably detect this tone on a composite multichannel optical signal. However, generally speaking, this modulation is not phase synchronized among channels and does not have a calibrated amplitude. The presence of a detectable Brillouin tone in the composite signal is therefore not guaranteed. In another approach, not admitted as prior art, one could overcome these problems by phase-synchronizing and calibrating the amplitudes of the Brillouin tones. To support subband protection, one could assign a different tone frequency to each subband. These modifications to the Brillouin tone scheme would, however, come at great expense due to the necessary adaptation of transponder hardware and software.
Another approach relies on injection of a special pilot tone on the protected multi-channel signal before the first amplifier in the protected link. This requires modification of certain amplifiers and a cumbersome differentiation between “first amplifiers” and other amplifiers. Also, in any WDM system that employs optical add-drop multiplexers (OADMs), the “first amplifier” will not be the same for all channels, precluding use of this scheme.
An approach described in D. Richards, et al., “Detecting Fiber Cuts in a WDM Ring with Optical Protection Switching: Simulation and Experiment,” (ECOC Proceedings September 1998) relies on comparison between a measurement of a signal received in a single marker channel added to support failure detection and measurement of a nearby “non-signal” region. The addition of the marker channel however reduces the number of channels usable to transmit data. Furthermore, the protection scheme becomes more complex when applied to optical subband multiplex section protection since each subband will require a marker channel. Also, wherever an OADM is used, the marker channel will need to be extracted and re-injected.
What is needed are systems and method for optical line failure detection that provide the needed speed, reliability, and accuracy while overcoming the drawbacks of the above-described approaches.